a. Field of the Invention
The invention relates generally to position servo systems of the kind used to position movable members, paricularly read/write transducers, relative to servo tracks on an information storage medium, such as a disk. In particular, the invention relates to a method and apparatus for determining the crossing of servo tracks while a transducer is traversing the tracks, when the position signal derived from the tracks drops out.
b. Prior Art
In accessing a desired servo track of an information storage medium or the like, a movable member such as a read/write transducer is usually moved across many tracks, each of which is counted, until the desired track is reached. Servo systems for accessing the desired track, as well as for maintaining the read/write transducer in the center of a desired track are termed position servo systems. It is known in the prior art that sawtooth waveforms may be obtained from a transducer crossing servo tracks and used for sensing transducer position in accessing a desired track. Such waveforms represent the position of the transducer or movable member relative to the next adjacent track. For example, see U.S. Pat. No. 3,737,883, entitled "Linear Positioning Apparatus for Memory Disk Pack Drive Mechanisms" by F. J. Sordello and I. W. Ha.
An example of an invention relating to accessing a target track on a disk, using a sawtooth position waveform may be found in U.S. Pat. No. 4,006,394, entitled "Coarse and Fine Control for Position Servo" by J. Cuda and F. J. Sordello, while an example of an invention relating to centering the transducer on a target track of a disk may be found in U.S. Pat. No. 3,864,740, entitled "Track Following Servo System", by the same inventors. The former patent teaches that a transducer velocity circuit, measuring the speed at which the transducer moves radially across the tracks, i.e., a tachometer, produces a velocity signal which may be integrated to yield a reconstructed position signal which signals the approach of a desired track. The latter patent teaches a typical prior art construction of a position servo loop.
Previously, as described in the aforementioned U.S. Pat. No. 4,006,394, a track crossing detector senses the crossing of a servo track by a movable member, such as a read/write transducer and provides an incremental count signal to a differential counter. This system is effective as long as the position signal can be detected.
Sometimes due to defects in the media on which the tracks are recorded, or other reasons related to noise, the position signal may not reach the extremities of the sawtooth waveform. In other instances the position signal, for various reasons, may simply momentarily disappear. These problems are termed signal "drop-out" herein. In these instances, the crossing detector which relies on this waveform will sense an incorrect track count and wrongly determine the position of the transducer.